The global positioning system (GPS) is a United States funded satellite system including twenty four satellites in a constellation that beams highly accurate timed signals to earth. There are other satellite systems which can be used for the same purposes. For instance, the GLONASS satellite system maintained by Russia also beams the accurately timed signals to earth. The generic term used for designation of any possible satellite positioning system is a satellite positioning system (SATPS). SATPS receivers can process the satellite signals from anywhere in the world.
With differential GPS, a stationary reference receiver is placed at a very accurately known point location. The reference receiver generates corrections which are sent to a transmitter, which in turn broadcasts the corrections to users within the area of the transmission broadcast. A differential GPS user receives these corrections through a radio/modem and applies them to the direct GPS measurements. This gives the user a position measurement of a very high accuracy, e.g., from one meter to ten meters. Differential GPS works quite well to produce meter-level accurate navigation as far as 100 kilometers from the reference station. Most radiolocation and radionavigation systems can be operated in a differential mode, and consequently provide improved accuracy. The features of continuous service, high effective update rate, and potentially large coverage areas make it possible for differential GPS to provide "real-time" positional information that could be obtained previously only in a "post-processing" mode of operation. This combination of capabilities of differential GPS makes it very attractive for a variety of applications.
The differential GPS finds application in the Marine Navigation. Indeed, in the restricted channels of some harbors and inland waterways 8-10 meters accuracy is required and the utilization of the differential GPS is essential to meet this requirement.
There is a general problem relating to all differential GPS receivers that is addressed in the present invention. The differences between the received and expected values are transmitted from the Reference Station to the GPS receiver over a separate communication link to correct the GPS receiver pseudorange measurements before the fix is computed. During the atmospheric noise, lightning, storms, weather anomalies, or electromagnetic interference the messages transmitted from the Reference Station to the GPS receiver can be damaged. Also, some differential GPS systems do not allow the use of error correction codes to correct the data errors resulting from the imperfect data transmission from the Reference Station to the GPS receiver. For instance, the Radio Technical Commission for Maritime Services (RTCM) doesn't recommend to use a self-correction code because it would increase the bandwidth required for the transmission.
Thus, what is needed is a correction algorithm which would allow the differential GPS receiver to process the damaged differential correction messages and to recover the useful information included in the damaged differential correction messages.